Findings Could Break Down Brick Wall Of Miniaturization

The important new results produced by the UD research team are “the ability to produce molecular wires at very precise positions, and to control the length of the wires through confinement inside of molecule corrals,” according to Thomas P. Beebe Jr., professor of chemistry and biochemistry.

Beebe said modern electronics, such as computer processors or the micro-circuitry that makes a cell phone fully functional, have hit a “brick wall” in attempts at further miniaturization. “Computer speeds have been increasing because engineers have been able to make the brains of computers, the transistors, smaller and smaller,” Beebe said. “The brick wall refers to the smallest possible building blocks for these transistors--atoms and molecules.”

Researchers have been predicting this size limit for the miniaturization of computer transistors due to the inherent limitations in the methods by which they are made, he said, explaining that computer transistors are made by techniques called lithography. These techniques utilize the properties of light or electrons to carve very small trenches, or “avenues,” into silicon computer chips.

In operation, electrons are the “cars” that travel from transistor to transistor along these avenues. The shorter the avenues, the faster the cars can get to their destination to carry out their function. Using lithography, there is an inherent limit to how small these features can be carved, and thus, to how fast they operate, Beebe said.

Because of this, Beebe said, many research groups all over the world have been pursuing alternate avenues by which to make smaller electronic devices. One of these avenues is the use of molecules, and these structures have come to be called molecular wires in the newly emergent field of molecular electronics. Molecules as wires would provide a major step forward in the miniaturization race, Beebe said.

Certain types of molecules called organic molecules consist mostly of carbon and hydrogen, he said. The carbon atoms can be connected with different types of chemical bonds, or electron clouds that surround atoms and hold one atom to the next in a chain of atoms. Certain types of chemical bonds have electrons that are also allowed to wander throughout the molecule. By making a molecule with several of these types of bonds and their wandering electrons, all connected in a line, it could be possible for these to function as molecular wires, Beebe explained. It is these types of molecules that scientists have focused on as they attempt to make wires out of molecules.

Beebe said one of the many challenges associated with the molecular wire idea is how to get the molecules to line up. “Since molecules are so small, it is not easy to physically move them on an individual basis, although this approach was tried over a decade ago by IBM scientist Don Eigler in what has become known as the world’s smallest advertisement,” he said. “Eigler used a microscope called the scanning tunneling microscope to push individual xenon atoms around on a metal surface to spell out ‘IBM’ using fewer than 100 atoms. Many scientists and engineers view this moment as the start of the fields of nanoscience and nanotechnology, from which the subfield of molecular electronics emerged.”

Beebe’s team, which includes UD chemistry and biochemistry graduate student Shawn Sullivan, postdoctoral researcher Albert Schnieders and visiting Lincoln University undergraduate student Samuel Mbugua, had been using the same type of scanning tunneling microscope as IBM’s Eigler used to study organic molecules.

For several years, this UD research team had been developing the tools to confine and line-up molecules in molecule corrals, a highly controlled nanostructure invented in the Beebe research group. Molecule corrals are pits approximately 1,000 times smaller than the diameter of a human hair, and they look like very flat moon craters on a very flat surface, Beebe said. The research team had been using these nanostructures to study the process by which molecules line up into highly ordered structures, a process called molecular self-assembly.

The Beebe team’s work built on findings by researchers Masakazu Aono and Yuji Okawa from Japan’s Institute of Physical and Chemical Research who, using a very complicated method of sample preparation, demonstrated the ability to line up their molecules by floating them on a water surface and then carefully transferring the lined-up molecules to a different surface for additional experiments. Once lined up perfectly, the scanning tunneling microscope was used to start a chain reaction between neighboring molecules. The Japanese researchers used the atomically sharp probe tip of their scanning tunneling microscope to initiate a chain reaction similar to the kind of reaction used to make polymers.

Beebe said that when the UD research team read the paper published by Aono and Okawa in the journal Nature in 2001, they had the idea to control the formation of molecular wires using molecule corrals. The first work from the UD research team was featured in the Feb. 15 issue of Langmuir.

Sullivan, the lead graduate student on the project, said, “The molecule corrals serve many purposes in this work, including that of reaction vessels, as boundaries to terminate the nanowire formation reaction, and as containers that can be filled with gold and silicon to potentially make a molecular nanocircuit.”

In addition, the methods of preparing lined-up or self-assembled molecules are much simpler and do not require expensive and complex equipment, Beebe said.

“We believe this work will be an important contribution to the field of molecular electronics as a means to produce new, smaller, faster devices that will lead further into the 21st Century and the era of nanotechnology,” Beebe said.

Mbugua, who was supported by the Howard Hughes Medical Institute as a visiting summer student from Lincoln University, said, “It was a great experience for me to be able to do research at the cutting edge with UD researchers.”

The research project is supported by funding from the National Science Foundation.